1. Field of the Invention
The present invention relates to a process for treating a sponge titanium powder to be used in a power metallurgy process as a starting material of a compact having titanium or titanium alloy.
2. Prior Art
Titanium or titanium alloy is an ideal metallic material to be used as a structural material because of its high toughness and superior specific intensity among metals practically used, in spite of a smaller specific gravity than steel. Furthermore, titanium or titanium alloy is also superior in corrosion resistance particularly to seawater and, therefore, this material has been widely employed not only for military weapons, aircraft and space rockets but also for private uses such as spectacle frames, golf goods, fishing rods, etc. In view of the superior characteristics such as light weight, toughness, corrosion resistance, titanium or titanium alloy is a material which is further expected in the future to play a more important role in modern industry.
However, to obtain a molded titanium product, it is necessary at present to install large-scale accurate equipment or plant and to carry out troublesome and skillful processes. The production of titanium and titanium alloy products can be broadly classified into two processes, i.e., a refining process which yields a sponge titanium and a later working process. In the former process, after producing a titanium tetrachloride by reaction between titanium ore (rutile) and chlorine gas and refining it, the titanium tetrachloride is reduced using metallic magnesium or sodium to obtain a sponge titanium. This metallic titanium is generally a sponge-like porous mass and, therefore, it is referred to as sponge titanium. The reduction method using metallic magnesium is called the Kroll process which is now most popularly employed. The obtained massive sponge titanium is then subjected to a crusher to be supplied in the form of grains to the next step, and in this crushing step, a fine powder thereof is selected as a by-product called sponge fine to be separately utilized.
The process up to the production of the sponge titanium requires a large amount of electric power even exceeding that for refinining aluminum. This required electric power occupies a significant percentage of the production cost of the sponge titanium, but it still remains in almost the same level as the material cost of ordinary stainless steel, heat resistant steel or the like. Therefore, a principal reason for the sharp increase in the production cost of titanium products depends largely upon the working process carried out later.
The reason for the sharp rise in price of finished titanium products amountings to 10 times as much as sponge titanium consists in the fact that titanium is by nature a material which is physically and chemically very active and, therefore, reaction thereof when contacting other components is very active throughout all the steps for forming a final product including dissolution, casting, forging, rolling and heat treatment, and that certain equipment for preventing pollution due to such active reaction and complicated procedures are essentially required, eventually resulting in abnormally high cost. To meet this problem of high cost, a molding by powder metallurgy not requiring any dissolution comes to attract one's attention, and in which so-called near-net-shape molding is possible to obtain a shape similar to that of the final product. This molding method has advantages such as improvement in material yield, large reduction in cutting and grinding cost, and therefore various developments have been designated to the application of this method for the purpose of largely reducing the production cost of titanium products to be put in practical use from an economical point of view.
The molding by powder metallurgy is classified into the blended element method and the pre-alloyed method. In either method, the starting material is a fine powder of titanium or titanium alloy and, therefore, the step for processing from a sponge titanium to a fine powder is indispensable. It is certain that the mentioned by-product obtained at the time of crushing a sponge is also a fine powder, but if using this by-product as it is as a starting material of the powder metallurgy method, there arises a disadvantage in a deterioration of material properties of the product, particularly a remarkable deterioration in the fatigue resistance characteristic. It has been acknowledged that such a decline in fatigue strength is caused by residual holes formed due to a chloride compound included in the powder material.
Metallic titanium has a characteristic of embrittlement when adsorbing or storing hydrogen. Thus, a hydrogeneration-dehydrogeneration method (HDH method) is also a popular method in which, utilizing the mentioned characteristic, a titanium hydride embrittled by hydrogeneration is crushed into a powder, which is then dehydrogenerated. This HDH method is widely adopted in this field of industry as one of the processes for efficiently obtaining a desired particle size of titanium or titanium alloy powder.
In a recent molding technique of titanium products by power metallurgy, a sponge titanium is transformed into a powder by the HDH method, and after sintering the power, a hot isostatic pressing (HIP) process is introduced for the purpose of collapsing the voids. Thus, it may be said that a titanium product obtained by the blended element method has reached the same level as a product obtained by melting and forging.
The process of the HDH method comprises the steps of hydrogenerating a sponge titanium; crushing; dehydrogenerating by heating and vacuum suction; sintering by heating; and cracking. To carry out this process a, large-scale equipmemt, long time, and much labor are required as a matter of course. Further, in the later process for obtaining a near-net-shape using the powder obtained in the former process as a starting material, the mentioned HIP process must be performed for applying a very strong pressure so as to collapse residual voids remaining in the mentioned powder. It is not an ignorable burden that such an intermediate process also requires large-scale equipment, a long time, and much labor. After all, it seems quite difficult to accomplish the object of sharply reducing the cost of titanium products and, therefore, it may be said that a primary factor of high cost restricting the usefulness of titanium still remains unsolved.
In view of the foregoing situation, an idea has been conceived in that a mechanical treatment is applied to obtain a powder suitable for the process of titanium by powder metallurgy. For example, the Japanese Laid-Open Patent Publication No. Hei 5-163508 discloses that, in the process for producing a titanium powder by the HDH method, an apparatus provided with some crushing means such as hammer crusher, hammer breaker, hammer mill is disposed, in substitution for a cutter mill conventionally disposed for cracking a sintered titanium mass after dehydrogeneration, to cut off sharp corner portions of every fine particle thereby obtaining a powder of desirable fluidity and high density suitable as a starting material for powder metallurgy. However, as far as this process employs the HDH method, it is doubtful that this process can bring about a remarkable cost reduction, i.e., it may be said that this prior method is yet insufficient for solving the mentioned problem of high production cost for titanium products.